Planets Space, 54, 907–916, 2002

Unusual ionospheric absorption characterizing energetic electron precipitation into the South Atlantic

Masanori Nishino1, Kazuo Makita2, Kiyofumi Yumoto3, Fabiano S. Rodrigues4, Nelson J. Schuch5, and Mangalathayil A. Abdu4

1Solar-Terrestrial Environment Laboratory, Nagoya University, Toyokawa 442-8507, Japan 2Basic Education Series, Physics, Takushoku University, Hachioji, Tokyo 193-8585, Japan 3Department of Earth and Planetary Sciences, Kyushu University, Fukuoka 812-8581, Japan 4INPE, Sao Jose dos Campos, Sao Paulo 12201-970, Brazil 5INPE, Southern Space Research Center, Rio Grade de Sul 97119-900, Brazil

(Received December 7, 2001; Revised September 25, 2002; Accepted September 25, 2002)

An imaging riometer (IRIS) was installed newly in the southern area of Brazil in order to investigate precipitation of energetic electrons into the South Atlantic Magnetic Anomaly (SAMA). An unusual ionospheric absorption event was observed in the nighttime (∼20 h LT) near the maximum depression (Dst ∼−164 nT) and the following positive excursion during the strong on September 22–23, 1999. The unusual absorption that has short time-duration of 30–40 min shows two characteristic features: One feature is a sheet structure of the absorption appearing at the high-latitude part of the IRIS field-of-view, showing an eastward drift from the western to the eastern parts and subsequent retreat to the western part. Another feature is a meridionally elongated structure with a narrow longitudinal width (100–150 km) appearing from the zenith to the low-latitude part of the IRIS field- of-view, enhanced simultaneously with the sheet absorption, and is subsequently changed to a localized structure. These features likely characterize precipitation of energetic electrons into the SAMA , associated with substorm occurrences during the strong geomagnetic storm. From the eastward drift (∼250 m/s) of the sheet absorption, precipitating electrons are estimated to be ∼20 keV energies, assuming plasmaspheric electric fields of 1.8 mV/m. However, no ionospheric effect due to the precipitating electrons was definitely detected by the ionosonde measurements at Cachoeira Paulista, separated eastward by about 1000 km from the IRIS station.

1. Introduction (22.7◦S, 45.0◦W), Brazil showed frequent nighttime occur- It is generally understood that quasi-trapped particles in rences of high blanketing frequencies ( fb Es). They inter- the inner radiation belt sink to the South Atlantic Magnetic preted as a regular nighttime ionization source in the SAMA. Anomaly (SAMA) which is characterized by a global min- Batista and Abdu (1977) exhibited significant enhancements imum in the Earth’s total magnetic field intensity. Possible in the sporadic-E layer parameters over the geomagnetic aeronomic effects by particle precipitation into the SAMA anomaly for short periods (2–3 hours) following moderate region were reviewed so far by several literatures (e.g., magnetic disturbances. Paulikas, 1975; Gledhill, 1976: Pinto and Gonzalez, 1989). On the balloon-borne experiments in the SAMA, Pinto On the ground-based observations, ionization effects in- and Gonzalez (1986) observed intensification of X-ray fluxes duced by precipitation of energetic electrons were inves- with energies between 30 keV and 150 keV in the atmo- tigated by broad-beam 30 MHz riometers and very-low- sphere during a strong geomagnetic storm using an omni- frequency (VLF) techniques. The riometer absorption at directional scintillation detector. They considered as the Atibaia in Brazil indicated an eastward drift of about 100 first evidence on the precipitation associated with the geo- m/s, associated with a sudden commencement followed by a magnetic activity inside the anomaly region. Jayanthi et al. strong geomagnetic storm (Abdu et al., 1973). Phase record- (1997) observed count rate increases of X-rays in the energy ings of 13.6 kHz Argentina VLF signals at Atibaia in Brazil range of 15 keV to 120 keV during a mild storm on a strato- showed significant perturbations due to the lowering of the spheric balloon experiment at L = 1.3 in the SAMA. VLF reflection level, associated with the onset of geomag- Using the Injun 3 measurements of 40 keV pre- netic disturbances (Abdu et al., 1981). Phase advances of the cipitating electrons, Torr et al. (1975) have theoretically es- 13.6 kHz Argentina VLF signals were linearly proportional timated that fluxes of precipitating electrons on L = 2in to a logarithmic function of the intensity of the geomagnetic the SAMA reach ∼35 times greater than the average flux on storm, given by the peak Dst (Pinto et al., 1990). Abdu and L = 2 in the northern hemisphere. From a four-year sur- Batista (1977) revealed from the ionosonde measurements vey by the Atmospheric Explorer Satellite (AE-C) over the that sporadic E-layer enhancements over Cachoeira Paulista SAMA, Gledhill and Hoffman (1981) demonstrated down- ward electron fluxes with low-energy (0.2–26.1 keV) for Copy rightc The Society of Geomagnetism and Earth, Planetary and Space Sciences 3 < K p < 6 in nighttime over the SAMA. Vampola and (SGEPSS); The Seismological Society of Japan; The Volcanological Society of Japan; Gorney (1983) calculated energy deposition profiles from The Geodetic Society of Japan; The Japanese Society for Planetary Sciences. spectra of locally precipitating 36 keV to 317 keV electrons

907 908 M. NISHINO et al.: UNUSUAL IONOSPHERIC ABSORPTION IN THE SAMA

Brazil, where the total intensity of the geomagnetic field is about 22,855 nT in 2000 (Private communication, Trivedi). In this initial paper we present an ionospheric absorption event observed by the imaging riometer during the strong geomagnetic storm on September 22–23, 1999, and discuss primarily whether the absorption feature is characterized by energetic electron precipitation into the SAMA.

2. Imaging Riometer An imaging riometer for ionospheric study (IRIS) which is capable of measuring shape, spatial scale and motion of ionospheric absorption was first developed at South Pole Sta- tion in the Antarctica in order to study dynamics of auroral particle precipitation in the high-latitude region (Detrick and Rosenberg, 1990). At present, multiple IRISs have been con- tinuously operated in the arctic region and in the Antarctica (e.g., Stauning et al., 1992; Nishino et al., 1993). Most of these IRISs adopt the antenna system consisting of a two- dimensional array with 8 × 8 half-wavelength dipoles with Butler-matrix phasing circuits at 38.2 MHz or 30 MHz, by which cosmic radio noises can be detected by an angular res- olution of half-power beam-width of 11◦. The fastest time- Fig. 1. The projection of half-power beam-width of two-dimensional 4 × 4 resolution of IRIS two-dimensional data is 1 second. Quiet beams to the ionospheric layer of 100 km altitude. day curves (QDCs) necessary to reduce ionospheric absorp- tion images from background cosmic radio noise are pro- cessed by a careful selection among about 10-days cosmic noise data on magnetically quiet days. Absorption intensities observed by the S3-2 satellite, and predicted that precip- are reduced by subtraction between decreased cosmic radio itating electrons into the SAMA could produce ionization noise and the QDCs. The QDCs are usually processed as the more than one order of magnitude greater than that expected monthly background. from scattered H Lymann α radiation at night. Kohno et al. The IRIS newly installed at the INPE-SSO consists of a (1990) obtained global distributions of energetic electrons two-dimensional array with 4 × 4 half-wavelength dipoles (0.19–3.2 MeV) from particle telescopes onboard the satel- at the operating frequency of 38.2 MHz. The separation be- lite OHZORA, in which electron fluxes were enhanced in the tween the dipoles is a half-wavelength. The two-dimensional altitude range of 700–850 km over the SAMA. array is aligned in the geographic north-south (N-S) and east- As described above, although there are many reports from west (E-W) directions, respectively. By the connection of the the ground-based, balloon-borne and satellite observations antenna array with Butler-matrix phasing circuits, the two- concerning particle precipitation into the SAMA region, dimensional 4 × 4 beams with a half-power beam-width of there remain still major uncertainties in understanding of dy- 22◦ are directed upward to the ionospheric area centered at namics in the inner radiation belt and precipitation processes the zenith. The projection of the two-dimensional 4× 4 half- of energetic electrons into the SAMA, as indicated by the re- power beams to the ionospheric layer of 100 km altitude is view paper of Pinto and Gonzalez (1989); (1) temporal and shown in Fig. 1. The field-of-view (FOV) of the IRIS is ap- spatial precipitation changes from magnetically quiet to dis- proximately 330 km in the N-S and E-W cross-sections near turbed periods (2) the role of wave-induced precipitation pro- the zenith. cesses. The recording system of two-dimensional 4 × 4 cosmic Recently, Badhwar (1997) has revealed that the center radio noise data using a personal computer (PC) is basically of the SAMA drifting westward is located at ∼30◦S and the same performance with the Ny Alesund˚ (NYA) IRIS ∼45◦W off the southeastern coastline of Brazil using does system (Sato et al., 1992). The two-dimensional 4 × 4 data rate data in June 1995 from the and orbital sta- are first converted to two-dimensional 8 × 8 ones by the tions. The total intensity of the geomagnetic field is near the use of interpolation and extrapolation calculations, and are global minimum of 23,008 nT according to the International led to the NYA IRIS procession system. Time variations of Geomagnetic Reference Field (IGRF) for 1990 (Takeda et 38 MHz radio noise and absorption intensities reduced from al., 1999). Therefore it has become more significant to rein- QDCs are printed as 8 channel stack plots for the N-S and E- vestigate particle precipitation in Brazil. Aiming at continu- W cross-sections (see Fig. 4 and Figs. 3 and 5, respectively). ous, long-term investigations of temporal and spatial changes The two-dimensional 8×8 data are further converted to two- of energetic electron precipitation into the SAMA from mag- dimensional 16×16 data in the PC in order to obtain smooth netically quiet and disturbed periods, we installed newly an absorption images with colors (see Plate 1). Thus absorption imaging riometer at the INPE-SSO (Instituto Nacional de images are apparently represented by spatial resolution of Pesquisas Espaciais, Southern Space Research Observatory, about 10 km × 10 km at the ionosphere of 100 km altitude geographic coordinate, 29.4◦S, 53.8◦W) near Santa Maria in near the zenith. M. NISHINO et al.: UNUSUAL IONOSPHERIC ABSORPTION IN THE SAMA 909

3. Observational Results LT) and decays at about 0015 UT on September 23. Compar- The IRIS observation at INPE-SSO has been carried out ing with the geomagnetic variation at Hermunus, it is found continuously since the middle in May 1999. From initial that the spike form absorption is observed near the maximum analysis of IRIS data during the first year, it has been tenta- depression and the following positive excursion. tively found that ionospheric absorption is usually observed In order to check the above absorption feature, we show in the time period from afternoon to evening with intensi- time variations of 38 MHz radio noises during 22 h–24 h ties of 0.2–0.5 dB during weak and moderate geomagnetic UT by shorter averaging-time of 16 seconds in Fig. 4, along storms (|Dst| < 100 nT). This occurrence characteristic with the monthly QDCs determined from variations of cos- is consistent with the daily variation of the ionospheric F- mic radio noise on geomagnetic quiet-days in September. region absorption associated with increases of fo F2 which Strong interference radio noise was received from the east- were obtained from the statistical analysis of the riometer ern part to the zenith during about 2250–2310 UT, result- absorption on 25 MHz at Ahmedabad in India (Abdu et al., ing in apparent zero absorption extended from the eastern 1967). Thus the absorption feature at INPE-SSO is identi- part. Thereafter strongest interference radio noise appeared fied as usual low-latitude F-region absorption. It has been almost in the whole FOV at about 2310 UT, and disappeared also found that the usual F-region absorption first appears temporally near 2322 UT and 2330 UT, resulting in impul- at the northernmost (low-latitude) part in the FOV, and is sive absorption features. When the strongest interference followed by expansion to the southern part (high-latitude) noise became weak simultaneously in the all beams at about through the zenith, depending on magnetic activities. Apart 2337 UT, background cosmic noise level started to decrease, from the usual F-region absorption, another absorption usu- which corresponds to the appearance of the spike form ab- ally appears at the northernmost part in the FOV with lo- sorption by longer averaging time of 128 seconds, as shown calized enhancement near local noon, which is identified as in Fig. 3. usual D-region absorption (Abdu et al., 1967). Figure 5 shows an enlarged time-variation of the spike In this section we present an ionospheric absorption event form absorption for the E-W cross-section at the southern observed during a strong geomagnetic storm occurred dur- (high-latitude) beam (N7), from which we can find an east- ing September 22–24, 1999, assigned as the ward shift between the E2 and E5 channels in the FOV, as is month. Figure 2 shows a daily variation of Dst (hourly displayed by a solid line, indicating an eastward drift of the value) during September 21–24, 1999 (courtesy of WDC- sheet absorption. In the following we show two-dimensional C2 for Geomagnetism, Kyoto University). A sudden com- images for the spike form absorption. mencement of the geomagnetic storm occurred at about 12 Plate 1 shows a time series of the absorption images from h UT on September 22, followed by positive fluctuation 2335:55 UT on September 22 to 0015:56 UT on September (Dst = 20 − 30 nT) during about 13 h–22 h UT and a subse- 23. Each absorption image is displayed as averaged during quent sharp decrease from about 22 h UT. The Dst attained 64 seconds. Corresponding to the spike form variation, sheet to the minimum value (−164 nT) near 0 h UT on September absorption first appeared in the southern (high-latitude) part 23, and recovered completely on September 25. at 2336:59 UT with mild enhancement showing an extension 3.1 The absorption event on September 22–23, 1999 toward the western area outside the FOV. The latitudinal Figure 3 shows 24-hour variations of the absorption dur- width is small-scale of about 150 km. Subsequently the ing September 22–23, 1999 in the upper panel, along with sheet absorption was enhanced showing the eastward drift a time variation of the geomagnetic horizontal component at to the eastern part until 2346:35 UT, and thereafter retreated Hermunus (geomagnetic lat., 33.7◦S), South Africa, in the gradually to the western part showing the extension to the lower panel (courtesy of WDC-C2 for Geomagnetism, Ky- high-latitude part outside the FOV. oto University). Each stacked point in the absorption is plot- In Plate 1, we find another feature of meridionally elon- ted as averaged during 128 seconds. The absorption varia- gated absorption with the longitudinal scale of 100–150 tions are displayed for N-S and E-W cross-sections centered km that was enhanced at the northern (low-latitude) part at near the zenith beam (N4E4). Note that N1–N4 and N5– 2338:03 UT. The enhancement was simultaneous with the N8 beams represent the directions to low-latitude and high- sheet absorption. The elongated absorption decayed at once latitude parts in the FOV, respectively. at 2341:15 UT, but thereafter enhanced again at 2343:23 UT Absorption increased on the all beams near 14h UT with a localized form. It should be noted that the localization (∼1030 LT) on September 22, followed by slowly varying within the one beam-width (Fig. 1) is caused by the effect of absorption until about 20h UT during the positive geomag- image procession, as described at the Section 2. The local- netic excursion. This absorption showed uniform expansion ized absorption stayed nearly in place until 2357:15 UT, and in the whole FOV. Thereafter the absorption fluctuated dur- thereafter elongated to the low-latitude part with gradual de- ing about 1940 UT to 2200 UT, and again slowly varied. cay. The absorption feature described above is identified as the As described above, the absorption feature shows unique usual F-region absorption during the mild geomagnetic dis- form and motion, and thus may be identified as an unusual turbance. Around 2240 UT the absorption suddenly disap- ionospheric absorption characterizing energetic electron pre- peared from the eastern part, and thereafter showed an im- cipitation into the SAMA. pulsive form, in particular, strong at the northernmost beam near 2330 UT. Here, it should be remarked that the absorp- 4. Discussion tion shows a spike form with strong intensity (maximum, Measurements of energetic particles in the inner radiation ∼1.2 dB at the N4E6 beam) after about 2337 UT (∼2000 belt have been carried out using some low-altitude satel- 910 M. NISHINO et al.: UNUSUAL IONOSPHERIC ABSORPTION IN THE SAMA

Fig. 2. A time variation of hourly Dst indices during September 21–24, 1999.

Fig. 3. Time variations of ionospheric absorption during September 22–23 in 1999 observed at INPE-SSO, Santa Maria in Brazil, along with the geomagnetic horizontal component at Hermunus (33.7◦S MLAT). The absorption variations are displayed on the N-S and E-W cross-sections centered at the N4E4 beam. Each stacked point is averaged during 128 seconds. M. NISHINO et al.: UNUSUAL IONOSPHERIC ABSORPTION IN THE SAMA 911

Fig. 4. Time variations of 38 MHz radio noise during 22 h–24 h UT on September 22 and the monthly QDCs determined from background cosmic noise on magnetically quiet days. The variations are displayed on the N-S and E-W cross-sections centered at the N4E4 beam. Each stacked point is averaged during 16 seconds.

lites. From the NOAA-6 satellite observations (h = 850 km) Kikuchi and Evans (1989) demonstrated that energetic electron fluxes (> 30 keV) of pitch angle 90◦ integrated over L = 1.3–1.4 increased dramatically than the previous average value near the maximum depression (Dst ∼−300 nT). Baker et al. (1994) observed counting rate of > 460 keV electrons near the magnetic equator by the CRRES , in which abrupt electron increases extended from L ∼ 2toL > 6 and often to the inner radiation belt of L < 2 during geomagnetic storms. Obara et al. (2000) displayed an evident increase of 30–1100 keV energy electrons in the inner radiation belt of L < 1.5 near the maximum depres- sion (Dst ∼−200 nT) during the strong geomagnetic storm from the NOAA-12 satellite. Thus it is expected that ener- getic electrons in the inner radiation belt could precipitate into the SAMA ionosphere during the strong geomagnetic storm, taking account of a numerical simulation of electron Fig. 5. Time variations of ionospheric absorption at the E-W cross-section at the southern beam (N7) during 20 h–04 h UT on September 22–23. pitch angle scattering in an asymmetric magnetic field envi- Each stacked point is averaged during 128 seconds. ronment (Abel and Thorne, 1999). 912 M. NISHINO et al.: UNUSUAL IONOSPHERIC ABSORPTION IN THE SAMA

Plate 1. A time series of the absorption images on September 22–23, 1999. Each image is displayed as the average during 64 seconds. Top is north, left is west. Absorption intensity is displayed by color-codes below. M. NISHINO et al.: UNUSUAL IONOSPHERIC ABSORPTION IN THE SAMA 913

We summarize characteristics of the present unusual ab- (−25 nT). Responding to this IMF change, step-wise de- sorption observed during the strong geomagnetic storm on creases on the geomagnetic X-components were recorded on September 22–23, 1999, as follows. the IMAGE Svalbard magnetometer network, and they at- tained to the maximum depression (−1000 nT ∼−1500 nT) 1) The geomagnetic horizontal component showed posi- at about 2145 UT (Luhr¨ et al., 1998), indicating substorm oc- tive short-period fluctuations and a subsequent sharp currences. Among the IMAGE Scandinavian magnetometer decrease with a few positive excursions, and attained to stations (60–72◦ MLAT), as shown in Fig. 6, the magnetic the maximum depression near 21 h LT. The unusual ab- X-components at the subauroral stations (OUJ, HAN and sorption was observed with short time-duration of 30– NUR) attained to the maximum depression at about 2230 UT, 45 minutes near the maximum depression and the fol- followed by some positive excursions in substorm recovery lowing positive excursion. phase. Therefore it is concluded that the unusual absorption ∼ 2) The unusual absorption showed two characteristic fea- commenced at 2330 UT is associated with the substorms tures: One feature is the sheet structure extended longi- prior to and near the maximum depression during the strong tudinally at the high-latitude part of the FOV, showing geomagnetic storm. an eastward drift from the western to eastern parts and Next we pay attention to the characteristic feature of the the subsequent retreat to the western part. Another fea- eastward drift of the unusual absorption appeared at the high- latitude part (Fig. 5). Here we estimate electron energies ture is the meridionally elongated structure with a lon- ∼ gitudinal small-scale width (100 ∼ 150 km) from the from the eastward drift velocity ( 250 m/s, assuming an zenith to the low-latitude part of the FOV and a subse- altitude of 100 km) of the sheet absorption. quent localized structure with gradual decay. Abdu et al. (1973) have inferred that the drift motion of the riometer absorption is subjected to an E × B drift in First we refer the geomagnetic conditions characterizing addition to the drift by the magnetic field gradient in the precipitation of energetic electrons from the past observa- plasmasphere. They gave the following relationship between tions in the SAMA region. Abdu et al. (1973) observed the observed drift rate of electrons (in degrees per second the unique absorption feature near 21 h LT in Brazil dur- that the guiding center of the particle makes at the center of ing the geomagnetic disturbance with the sudden commence- the earth) and the drifts due to the magnetic gradient and the ment followed by the positive short-period fluctuations, the E × B force: sharp decrease, the maximum depression and the recovery. (58/R L)(d/dt) = (6/44)Le − 58(E/B R )L2. The unique absorption with time-duration of 30–60 minutes 0 eq 0 corresponded to the time period of the positive excursions d is ionospheric separation between the axes of the two an- around the maximum depression. This geomagnetic varia- tenna beams, and dt is the time difference, and thus (d/dt) tion is similar to that on the present unusual absorption. corresponds to the velocity of the absorption drift in the ab- On the other hand, Batista and Abdu (1977) demonstrated sorption altitude. L is the corresponding magnetic shell pa- significant enhancements on the Es -layer parameters in the rameter, R0 is the radius of the earth, e is the electron energy SAMA region during the time-periods (2–3 hours) after the in MeV, Beq is the surface magnetic field intensity and E moderate magnetic storm. Abdu et al. (1981) demonstrated is the electric field, which here is considered positive if di- enhancements of the Es -layer parameters and lowering of the rected upward in the equatorial plane and negative if down- VLF reflection level in the SAMA region with some delay ward. Giving L = 1.2 and Beq = 0.312 Gauss, and as- with respect to the magnetic disturbance onset. They sug- suming electric fields, we can estimate electron energy from gested some degree day-to-day variability in the abundance the above relationship. Here we refer upward plasmaspheric of metallic species and/or in the dynamics of the E-region electric fields of 1.8 mV/m mapped to the equatorial plane over the SAMA. near 23 h MLT at L ∼ 1.6 which was measured by the S3-3 Pinto and Gonzaletz (1986) observed an intensification of satellite during strong geomagnetic activity (|Dst| > 100 nT) X-ray fluxes between 30 and 150 keV in association with a (Gonzalez et al., 1986). Then we obtain electron energies of strong geomagnetic storm (maximum |Dst|=291 nT) by 16.2 keV. If we assume 10% error for the velocity, electron the balloon-borne experiments in the SAMA. Jayanthi et energy is in the range of 15.1 keV to 17.4 keV, which could al. (1997) observed cosmic X-ray fluxes in the energy range cause maximum ionization at the lower E-region of ∼100 between 18.6 to 120 keV by the balloon-borne experiment km altitude (Rees, 1963). during a mild storm. They suggested the diffusion of par- Thus we infer that energetic electrons are injected from ticles from higher L regions into flux tubes connected the the nightside plasma sheet associated with the substorm suc- SAMA region, due to electric field fluctuations associated cession from 20 h UT, which are represented for the posi- with succession of substorms. As described above, the geo- tive short-period geomagnetic fluctuations during the storm magnetic conditions characterizing particle precipitation into initial phase. The energetic electrons should drift eastward the SAMA region are not necessarily definite. Here we in- encircling the Earth, and thereafter the electrons would pre- vestigate substorm occurrences with regards to the present cipitate into the SAMA ionosphere associated with the geo- unusual absorption event. magnetic positive excursions near the maximum depression Around 2000 UT on September 22 when was about 3.5 of 0 h UT. This inference is a little different with the sugges- hours prior to the unusual absorption, the Bz-component tion by Trivedi et al., (1973) that the short-period magnetic of the interplanetary magnetic field (IMF) from the IMP- fluctuations are directly responsible for higher-energy (> 1 8 satellite suddenly and largely changed to the southward MeV) precipitating flux, which could cause the D-region 914 M. NISHINO et al.: UNUSUAL IONOSPHERIC ABSORPTION IN THE SAMA

Fig. 6. Geomagnetic X-component variations on September 22–23, 1999 at the Scandinavian stations in the IMAGE magnetometer network. The variations are plotted by 1 min average. ionization enhancement. 26.1 keV) with the ranges of 2–3◦ in latitude and 1–2◦ in In Plate 1, we notice that the sheet absorption is enhanced longitude on one over the SAMA region from the AE- simultaneously with the meridionally elongated absorption C satellite observations. Pinto and Gonzalez (1986) detected at 2338:03 UT. The enhancements repeat at 2343:23 UT and an intensification of the X-ray fluxes (30–150 keV) in the 2348:43 UT with the drift motion, and thereafter retreat to area covering a longitudinal interval < 4◦ at L = 1.13 from the western part in the FOV. This behavior should be com- the balloon experiment. Recently Heirtzler and Allen (2002) pared with the calculation results by Torr et al. (1975): As- showed a global distribution of flux of trapped electrons of suming that the mirror height is 100 km at L = 1.8inthe > 300 keV measured by the NOAA-11 satellite at an altitude southern hemisphere, they have revealed that precipitated of 815 km during the greatly disturbed period of March 11– fluxes for 280 keV electrons show a sharp increase (> 35 20, 1989, in which we can find strong fluxes in the area of times greater than the average flux) with very narrow lon- several hundred kilometers centered near Santa Maria. Actu- gitudinal width near 30◦WatL = 1.5, and show a dras- ally the strong fluxes may be distributed at the smaller area. tic decrease eastward than 30◦W, implying that stored elec- Gonzalez et al. (1987) have considered theoretically that the trons are drastically precipitated near 30◦W. Although the intensification of precipitation could occur in fairly narrow estimated electron energies (∼20 keV) and the concerned region of the order of a few hundred kilometers at low L L-value (L = 1.2) for the present unusual absorption are values (L = 1.13), using a numerical simulation of mid- in some degree different with the calculations by Torr et dle atmospheric ionization effect in the atmospheric electric al. (1975), it is qualitatively understood that the present en- field at stratospheric levels. These observations and the sim- hancements would be explained as the intensification associ- ulation may suggest plausibility of the small-scale enhance- ated with the sharp increase of the precipitation fluxes, which ments of the present unusual absorption. was induced by the substorm succession during the strong Finally, in Fig. 7, we show time variations of the iono-  geomagnetic storm. spheric parameters ( fo F2, h F2 and fo Es ) on September 22– In the following we discuss the small-scale widths (100– 23, 1999 measured by the ionosonde at Cachoeira Paulista 150 km) of the meridionally elongated and the localized ab- (22.7◦S, 45.0◦W) in Brazil, in order to check the ionospheric sorption displayed in Plate 1. Gledhill and Hoffman (1981) effect due to electron precipitation. Unfortunately Cachoeira demonstrated a sharp peak of downward electron fluxes (0.2– Paulista is separated eastward by about 1000 km from the M. NISHINO et al.: UNUSUAL IONOSPHERIC ABSORPTION IN THE SAMA 915

 Fig. 7. Time variations of ionospheric parameters of fo Es (in frequency), fo F2 (in frequency) and h F2 (in altitude) on September 22–23, 1999 measured by the ionosonde at Cachoeira Paulista in Brazil. The parameters are measured every 15 min.

 IRIS station. In Fig. 7, the h F2 shows a step-wise increase positive excursion during the strong geomagnetic storm, at about 14 h UT, which may correspond to the step-wise ab- which indicates the association with the substorm occur- sorption increase started at just before 14 h UT in Fig. 3. The rences. The absorption showed the two characteristic fea- fo F2 increase during 15 h–18 h UT may indicate the usual tures: One feature is the sheet structure absorption appearing low-latitude F-region absorption from 15 h UT in Fig. 3, be- at the high-latitude part with the eastward drift and subse- cause the absorption images show expansion in the whole quent retreat. Another feature is the meridionally elongated FOV. Thereafter the fo F2 shows a large, broad increase at- structure absorption with the longitudinally small-scale, en- taining to the frequency of ∼17 MHz at about 21 h UT, al- hanced simultaneously with the sheet absorption. These  though the h F2 is not so high (250–300 km). This represents features reveal that the eastward drifting electrons injected the ionospheric disturbance expanded over the large area in from the nightside plasma sheet would precipitate into the low-latitudes during the main phase to recovery phase. The SAMA ionosphere in asymmetric geomagnetic environment strong extraneous interference noise extended from the east- between the northern and southern hemispheres. However, ern part in the FOV (Fig. 4) must be caused by the iono- it needs more discussions of precipitation processes after spheric disturbance. further identifications of unusual absorption events during It should be remarked in Fig. 7 that the fo Es shows an ab- strong geomagnetic storms. Simultaneous satellite particle normal peak exceeding 6 MHz between 2145 UT and 2315 data will be helpful for discussions of dynamics of energetic UT. If the electrons of ∼20 keV energies precipitate, the electrons in the inner radiation belt. fo Es peak would be appropriate. However, the facts that the fo Es peak is advanced by about 1 h comparing to the Acknowledgments. This project is progressed in collaboration time-period of the unusual absorption and also that the un- with Instituto Nacional de Pesquisas Espacias (INPE) and Federal University of Santa Maria in Brazil. This work is partly supported usual absorption could not drift eastward outside the IRIS by Monbu-Kagaku-Syo International Scientific Research Program. FOV (Plate 1) seem to be unreasonable. Consequently the One of authors, K. Makita, was supported by Science and Engi- unusual absorption characterizing energetic electron precip- neering Institute in Takusyoku University. We thank Y. Kato and itation over the IRIS station was not definitely detected by M. Sato, STE Laboratory, Nagoya University, for the production the measurements of ionospheric parameters at Cachoeira of the IRIS’s antenna and the data procession system. The antenna installation at the INPE-SSO is assisted by students in Laborato- Paulista. rio de Ciencias Espaciais de Santa Maria, to whom we thank very much. The geomagnetic data are provided by WDC-C2 for Ge- 5. Concluding remarks omagnetism, Kyoto University. The IMF data and IMAGE chain We have presented the unusual absorption event observed magnetic data are provided through the web site, to which we thank at night in the SAMA region. The unusual absorption was very much. observed near the maximum depression and the following 916 M. NISHINO et al.: UNUSUAL IONOSPHERIC ABSORPTION IN THE SAMA

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